Delta T

[Richard Thacker]

At what outdoor temperatures will you see a heat pump begin to have difficulties achieving a typical temperature differential (Please state your temperature ranges you use when testing a system). http://needahomeispector.com (http://needahomeispector.com/)

[Jim Luttrall]

Can't give you solid statistics on that.
In general heat pumps run out of gas (figuratively) at around freezing and in the low 20's you get down to spending about as much to run them as you get heat out. I never use a hard and fast delta T but a sliding scale depending on indoor temperature, outdoor temperature, age of the system, etc. There are too many variables to give a unit a pass or fail based on a delta T.
Then you can get into the newer two stage pumps designed to run in cold temps without supplemental.
By the way, we never size our heat pumps based on heating load, that is done with the cooling load and whatever heat you get just comes along for the ride with supplemental heat making up the difference.

[Richard Thacker]

Thanks